Screw-driven systems for effecting a desirable pressure change in a braking system are well known devices. U.S. Pat. No. 4,653,815 which issued Mar. 31, 1987 describes one such device. The use of such a motor driven screw to effect translation of a piston that in-turn, effects a corresponding increase or decrease in fluid pressure contained within a closed braking circuit is well known.
In using such a conventional screw-driven system for effecting brake pressure modulation in wheel brake applications or releases, resulting system pressure gradients are important. A significant amount of development has concentrated on provided a motor actuator that operates to enable generation of the pressures and response times required while meeting energy limitation requirements using the now conventional screw-driven technology.
Such motor actuated screw-driven systems have proven particularly successful for use in the application and release of fluid pressure in braking systems. However, this success has resulted in the use of motor actuators that require significant amounts of energy to operate and are somewhat costly.
Electrical power consumption has become increasingly more significant as braking systems find application in electric powered vehicles in addition to the more prevalent internal combustion engine powered vehicles. Accordingly, improvements in screw-driven systems generally, and for use in vehicle braking systems in particular, would benefit from potential improvements resulting in possibly lower energy consumption requirements.
In the pressurization of a braking system during full braking cycles, it has been found that the actual forces required of the screw-driven system are variable. During initial braking system pressurization, low forces are encountered as brake pads begin to contact their corresponding rotor or shoe and as compliance of hoses in the braking lines is accounted for. During this period of brake application, high fluid flow conditions under relatively low pressures are encountered. Subsequently, as the brake pads begin to fully contact the rotors or drums and compliance is overcome, relatively high pressures, often exceeding 2000 pounds per square inch, are encountered. During this latter period of brake application, relatively lower amounts of fluid flow are required.
In order to meet the requirements for these two functionally different periods of brake application, a screw-driven system must be capable of effecting translation of the piston in a manner resulting in high fluid flow conditions under relatively low pressure while at the same time operating under lower fluid flow conditions and significantly higher pressures.